The present invention relates to sensors and particularly, to a sensor with a metal oxide coating for resistance to poisoning.
The automotive industry has used exhaust sensors for many years to detect the composition of exhaust gases such as oxygen, hydrocarbons, nitrous oxides and the like. For example, an oxygen sensor measures oxygen concentration in exhaust gases post combustion. This information is then utilized to control the operation of the engine, i.e. control air to fuel ratios, to enable improved performance and/or reduced emissions.
In a conventional sensor, the sensor comprises a first electrode capable of sensing an exhaust gas and a second electrode capable of sensing a reference gas with an ionically conductive solid electrolyte disposed therebetween. High temperatures and materials such as silicon, lead, and the like, present in engine components, can poison or otherwise damage the sensing electrode. In order to prevent poisoning/damage to the sensing electrode, a protective layer made of spinel or the like, has conventionally been applied to the sensing electrode. At protective layer thicknesses which retain the electrode performance, the silicon, lead, and other particles can pass through the layer and poison the electrode. At higher thicknesses which more effectively inhibit transmission of the poisoning material(s), the efficiency of the electrode is decreased. Furthermore, although the protective layer inhibits access of the poisoning particles to the electrode, the protective layer itself can become clogged, thereby also inhibiting passage of exhaust gases for sensing.
One conventional poison resistance technique comprised applying multiple layers of a heat-resistant metal oxide(s) over the electrode to form a protective layer. However, the multiple layers changed the performance of the sensor and only provided limited poison protection.
Accordingly, there remains a pressing need in the art for a sensor that is poison resistant for a longer period of time without any significant performance change.
The present invention relates to a sensor and a method for making the same. The sensor comprises: a sensing electrode having a first and second side and a first electrical lead in electrical communication with said sensing electrode; a reference electrode having a first and second side and a second electrical lead in electrical communication with said reference electrode; an electrolyte disposed between and in intimate contact with said first side of said sensing electrode and said first side of said reference electrode; and a first side of a protective layer disposed adjacent to said second side of said sensing electrode, wherein a second side of said protective layer has a protective coating, said protective coating comprising a mixture of alpha alumina and gamma alumina.
The method for making the sensor comprises: using a sensing electrode having a first and second side and a first electrical lead in electrical communication with said sensing electrode; using a reference electrode having a first and second side and a second electrical lead in electrical communication with said reference electrode; disposing an electrolyte between said first side of said sensing electrode and said first side of said reference electrode; disposing a first side of a protective layer adjacent to said second side of said sensing electrode; and disposing a protective coating comprising a mixture of alpha alumina and gamma alumina in physical contact with said protective layer.